Transform coding is an efficient image compression scheme that typically involves segmenting a picture, or image, into blocks of pixels, taking discrete cosine transforms ("DCTs") of the blocks of pixels to obtain blocks of DCT coefficients, quantizing these coefficients, and coding the quantized coefficients by an entropy coder. Interframe coding schemes utilizing motion compensation and transform coding of motion compensated interframe differences, by taking DCTs of blocks of difference pixels, quantizing the DCT coefficients and entropy coding the quantized DCT coefficients, may also be employed.
Interframe coding employing motion compensation and DCT coding has become widely recognized as a particularly efficient coding scheme for video compression and forms the core of the Comite Consulatif International Telegraphique et Telephonique Recommendation H.261-Video Codec for Audiovisual Services at 64 Kbit/s, Geneva, August, 1990 ("CCITT H.261") and the Motion Pictures Expert Group Phase 1 ("MPEG-1") video compression standards. The MPEG-1 standard is set forth in International Standards Organization ("ISO") Committee Draft 11172-2, "Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to 1.5 Mbits/s," November, 1991. The CCITT H.261 standard primarily addresses coding of video conferencing scenes of Common Intermediate Format resolution at bitrates of 64 kbit/s to 2 Mbit/s; the MPEG-1 standard can be efficiently used for coding all types of video scenes in the 1 to 2 Mbit/s range. The MPEG-1 standard incorporates additional features, for example, group-of-pictures concepts, motion compensated bidirectional prediction, and variable word-length coding. Variable word-length coding, which is also known as Huffman coding, is typically employed after the DCT portion of the encoding process. Each DCT event, comprised of run-length and level, is represented by a codeword whose length depends upon the relative value of the coefficient. Longer word lengths may be used to represent higher valued coefficients while shorter word lengths may be used to represent coefficients having a value of zero or close to zero. MPEG-1 supports the coding intra-coded ("I-"), predictive-coded ("P-"), and bidirectional predictive ("B-") pictures. Unfortunately, because the MPEG-1 standard was initially applied in the coding of low resolution video, it is not fully optimized for coding of higher resolution video sources such as interlaced video conforming to the Comite Consultatif International des Radiocommunications Recommendation 601 and the high definition television format.
Consequently, many improvements to video coding have been developed since the adoption of the MPEG-1 standard. One such improvement is discussed in U.S. Pat. No. 5,227,878 by A. Puri et al. which includes reference to adaptive word-length coding. The use of such a variable word-length coding scheme produces a substantial improvement in coding efficiency.